In general, the wheels of in-line skates are mounted one behind the other on the side edges of a plate secured to a boot. Each wheel is mounted so that it can rotate on a fixed axle by means of two rolling bearings, the said axle being secured by its ends to the edges of the plate.
The document DE-U-297 08 535 relates to in-line skate devices equipped with instrumentation. The back wheel of a skate is equipped with an encoder which moves in rotation past a sensor secured to a non-rotating part of the skate. The signal is processed and broadcast wirelessly to a display bracelet located on the skater's wrist. However, since the wheels are level with the ground, the device for detecting rotational speed is particularly exposed to different kinds of contamination by water, mud and other dirt. The skates are also subject to frequent impacts caused by the skater's falling, jumping curbs, etc. The aforementioned document does not discuss this subject.